Sol-gel technology involves the preparation of ceramics and glasses by hydrolysis and condensation of metal oxide precursors (usually organometallic compounds) in solution. In general, this approach reduces the sintering temperature required to obtain the metal oxide ceramic. Furthermore, sol-gel technology is an economical method of fabricating thin films. However, the subsequent processing after application of the sol-gel thin film can involve failures, cracked films, pinholes, etc.
Vapor deposition methods, such as evaporation, sputtering, and chemical vapor deposition (CVD), are methods commonly used in fabricating semiconductor devices, including thin film resonator devices such as resonators, filters, oscillators, etc. However, sol-gel techniques can lead to more conformal coverage of the substrate than can be obtained by vapor deposition methods.
According to a typical method for fabricating, for example, a semiconductor device, a thin film of aluminum is deposited onto a layer of silicon dioxide formed over a silicon wafer substrate. The aluminum film is masked by photoresist and then etched in a plasma to form an electrode or conductor pattern.
In the step in the fabrication of a semiconductor device involving formation of a conductor geometry by etching a thin layer of aluminum or aluminum alloy, the alloy covers an underlying layer of the semiconductor device. The underlying layer may be silicon dioxide. The conductor pattern to be etched from the aluminum is defined by a photoresist masking layer formed over the aluminum such that only regions unmasked by the photoresist will be etched. To achieve satisfactory results, it is required that the etching proceed only in the desired areas and that it not otherwise affect the integrity of the remaining portions of the layer.
Commonly, thin film resonator devices are produced by fabricating p.sup.+ membranes in the surface of the silicon wafer by means of boron diffusion into the wafer, followed by backside etching of vias through the bulk silicon wafer to the p.sup.+ membrane etch stop, and thereafter placing the desired circuitry over the membrane which is then removed. However, the use of .sup.+ membranes in thin film resonator technology fabrication has a detrimental effect on the silicon substrate resulting in poor reproducability and low product yield. Moreover, when a boron p.sup.+ etch stop is used by depositing the same over a silicon dioxide surface grown on top of a silicon wafer, this must be followed by a deglazing sequence, after the etching in order to remove the resulting boro-silicate glass (xB.sub.2 O.sub.3 .multidot.ySiO.sub.2).
The commonly used etch material used to etch the backside of the wafer is ethylenediamine-pyrocathechol-water (EDPW).
Since solutions of EDPW etch silicon at a rate much faster than that of silicon dioxide, a second oxidation is necessary to grow a new layer of thermal silicon dioxide on both the front and back of the wafer in preparation for backside lithography. Once the back of the wafer is patterned, the thermal silicon dioxide layer acts as a mask so that the vias can be etched through the bulk of the silicon wafer with a hot EDPW solution, with the etch stopping at the boron p.sup.+ membrane. However, as earlier stated, the high boron concentrations in the silicon crystal lattice lead to misfit dislocations which can not only be detrimental to the production of the active devices, but are also known to contribute to low yield and poor reproducability of the process.
Accordingly, it is a primary objective of the present invention to first build the devices on the silicon wafer, to then employ a sol-gel derived tantalum oxide film as a layer over the device, and to etch without using a p.sup.+ membrane. The result is the building of thin film resonator devices quicker, cheaper, of higher integrity, and of greater yield without the use of p.sup.+ membranes, by a process which allows etching of the backside vias as the final processing step.
The method and manner of accomplishing each of these objectives will become apparent from the detailed description of the invention which follows hereinafter.